Phototube having a photocathode adapted to absorb substantially all the light energyreceived



Jan. 17, 1967 N. s. KAPANY 3,299,306

PHOTOTUBE HAVING A PHOTOCATHODE ADAPTED TO ABSORB SUBSTANTIALLY ALL THE LIGHT ENERGY RECEIVED Filed July 25, 1964 2 Sheets-Sheet S E es 63 63 Fl 6.?

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62 NARINDER s. KAPANY BY 6 65 fi W ATTORNEYS United States Patent PHOTOTUBE HAVING A PHOTOCATHODE ADAPTED T0 ABSORB SUBSTANTIALLY ALL THE LIGHT ENERGY RECEIVED Narinder S. Kapany, Woodside, Calif., assignor to Optics Technology, Inc., Belmont, Calif., a corporation of California Filed July 23, 1964, Ser. No. 384,667 13 Claims. (Cl. 313-95) This invention relates in general to phototubes and more particularly to photon electron multipliers producing a fast output response to a light signal and utilizing substantially all the energy of the incident light signal.

In the handling of light of relatively low intensities, phototubes are employed to produce an output signal in response to detected light, The conventional phototube comprises a photosensitive cathode which, when irradiated, gives off electrons which flow to an anode. The anode can either then provide an electrical signal or a reconversion of the electrons to light energy by the use of a phosphorous coating which will cause luminescence in proportion to the amount of electron bombardment received.

Because of the low incident light energy often available, it is desirable in phototubes to be able to utilize all the light energy as efliciently and effectively as possible. If substantially all light received from a given light source cannot be converted into electron energy in the photocathode of the phototube, either no signal or a signal of insuflicient amplitude results.

In the past, the optical structures for directing light as received from a source to the phtotocathode of a phototube have permitted either a certain portion of the incident light energy to be reflected back out of the system or a portion to be passed through the photocathode on into the phototube wherein the light did not aid in the production of an output signal. Thus, only a portion of the light energy incident on the photocathode was converted to electron energy. In order to compensate for the low photoelectric sensitivity of these prior art photocathodes and to provide gain where a certain output level was required multiplier phototubes have been utilized. These photomultiplier tubes include one or more secondary-electron-emitting electrodes or dynodes between the photocathode and output electrode. While a suflicient output signal can ultimately be produced with such a photomultipler arrangement, the transit time of the electrons between dynodes reduces the effectiveness of the apparatus.

The present invention is directed to a phototube in which substantially all of the light energy received from a source is to be absorbed by the photocathode, and, where necessary, secondary electron emitting electrodes are closely spaced from the photocathode so that within the smallest space possible an output signal of the desired level is rapidly produced in response to an input light signal that may be very weak.

One of the principal objects of this invention is to employ in a phototube a photocathode for-med to accomplish substantially complete absorption by total internal reflections of the incident light and further-more to produce a plurality of passes of the incident light within the cathode.

A further object of this invention is to provide in a phototube a light transmitting and absorbing cathode layer bounding at least a portion of a first medium with means for introducing light through the first medium and into the cathode to be reflected in the cathode and with the boundaries of the supporting medium arranged to redirect light that is reflected from the cathode back into the 3,299,306 Patented Jan. 17, 1967 ice cathode again for reflection therein. The thickness of the cathode is selected to permit high probability of escape for the electrons and absorb substantially all the light energy in the incident beam whether at the high or the low end of the spectrum.

A feature and advantage of this invention lies in the fact that total internal reflection directs substantially all of the incident light to the cathode to obtain extremely high efliciency of photon to electron conversion.

Another object of this invention is the provision in such a phototube of a focusing reflecting surface at the boundary of the medium on which the cathode is supported for reflecting light of different angles of incidence onto the cathode such that additional reflection takes place.

A further feature and advantage of the invention lies in the fact that the cathode supporting medium will accept non-collimated light from a source such as, for example, an optical fiber and still produce total internal reflections for eflicient conversion of light energy to electrical energy.

A still further object of this invention is to provide in a phototube an improved photocathode light-collecting mechanism which incorporates a tubular layer of a lighttransmitting and absorbing cathode material supported on one light-transmitting medium and having an emission interface with another medium which has a refractive index lower than that of the cathode material and providing means for directing usable light input into the one light-transmitting medium such as to penetrate the cathode material and be reflected therein without any light passing into the other medium.

A still further feature and advantage of this invention lies in the fact that multiple internal reflections take place within the photocathode for emission of electrons from the photocathode radially outwardly and light reflected from the cathode emission surface at one side of the cathode is reflected to the other side of the cathode where another portion can be absorbed and the remaining light reflected back again to the first side until the light is substantially entirely absorbed.

Another object of the present invention is to provide means for introducing the incident light into one end of the one light-transmitting medium or optical waveguide at angles up to and including the incidence angle da which is determined from the following formula:

max= sin dag 1/2 where n is the refractive index of the medium at the input face of the optical waveguide, n is the refraction index of the optical waveguide, and n is the refractive index of the other medium at the cathode emission interface.

Another feature and advantage of the invention lies in the fact that light introduced into the optical waveguide at acceptance angles up to and including the angle passes into and through the photocathode for reflection and absorption therein without passing into the other medium.

Another object of the present invention is to employ in combination with such a photocathode a surrounding cylindrical secondary-electron-emitting electrode coaxially mounted with respect to the tubular cathode and maintained at a more positive potential than the cathode.

An additional feature and advantage of this invention lies in the fact that the electrons generated at the photocathode surface move radially outwardly from the cathode and strike the surrounding cylindrical secondary-electronemitting electrode after a short transit time. Thus, a compact structure is provided that does not require complex shapes to focus the electrons from the cathode to the secondary-electron-emitting electrode.

This photomultiplier combination is capable of achieving complete absorption of incident light and subsequent amplification over a broad frequency band.

Another object of the invention is to employ in a photo multiplier, including a cylindrical photocathode, a surrounding cylindrical secondaryelectron-emitting electrode coaxially mounted with respect to the cathode which is mounted at one end of the electrode, an anode arranged at the other end of the secondary-electron-emitting electrode and transverse to the axis thereof and means for maintaining a potential gradient along the secondaryelectroncmitting electrode in the direction from the cathode end to the anode end so that an increasing number of electrons are secondarily emitted along the length of the electrode in the direction of the anode when electrons are generated from the photocathode to bombard the secondary-emitting electrode.

A still further feature and advantage of this invention lies in the fact that an amplified signal can be achieved utilizing the cylindrical cathode in a structure having a minimum diameter thereby permitting an array of such amplifying elements to be mounted in a small volume.

Another object of the present invention is to provide in a photomultiplier, having a tubular photocathode, cylindrical transmissive secondary-electron-emitter electrodes surrounding and coaxial with the cathode and an anode electrode surrounding and coaxial with the secondary-electron-emitting electrodes whereby electrons emitted from the cathode travel radially outwardly to the first secondary emitter at which electrons are generated substantially radially outwardly toward the anode so that after generation of electrons at successive secondary emitters a large signal appears at the anode.

Still another advantage of this invention lies in the fact that the electrodes are conveniently supported coaxially and the number of electrons produced in the successively radially outward electrodes increases as the areas of the electrodes increase.

Still another advantage of this invention lies in the fact that this transmissive secondary-electron-emitter electrode construction produces a fast response due to a fast transit time and a short transit time dispersion.

Another object of the present invention is to provide a phototube employing a light-transmitting fiber having a light-receiving end and a light-transmitting and absorbing cathode layer surrounding at least a portion of the fiber with an anode electrode surrounding the fiber and providing means for introducing light into the light-receiving end of the fiber to penetrate the cathode and strike the emitting surface of the cathode without passing to the anode.

Still another feature and advantage of the invention lies in the fact that in such a phototube the light-transmitting fiber allows incident light energy to reflect on the surface of the cathode a plurality of times in a short length of fiber so that, upon each passage of the light through the cathode, energy is absorbed for electron generation with the electrons moving radially outwardly to the surrounding anode, thus providing an extremely efficient energy converter. Such a construction results in a detector having a fast response, high sensitivity, and high electronic gain within a small volume.

A still further object of the present invention is to provide a phototube including a plurality of closely spaced but optically and photoelectrically insulated photocathode surfaces arranged within a coaxially arranged secondaryelectron-emittin-g electrode structure and anode structure, the surrounding electrode and anode structure divided into a like plurality of segments electrically insulated from one another, each segment arranged to produce an output response to the electrons generated from only one of said photocathode surfaces.

A still further feature and advantage of this invention lies in the fact that a compact optical light source tracker is provided in a minimum amount of space but with a high efliciency and fast rseponse over a broad band.

Other objects of. the present invention will become apparent upon reading the following specification and referring to the accompanying drawings in which similar characters of reference represent corresponding parts in each of the several views.

In the drawings:

FIG. 1 is a schematic side sectional view of a detector embodying features of the present invention;

FIG. 2 is a schematic side sectional view of an alternative structure in accordance with the present invention;

FIG. 3 is an enlarged view of a portion of the structure shown in FIG. 1 illustrating the operation of the present invention;

FIG. 4 is a side sectional view of an alternative embodiment of the present invention;

FIGS. 5 and 6 are schematic side sectional views similar to FIG. 4 but of still other alternative embodiments of the present invention; and

FIG. 7 is a schematic end sectional view of another structure employing the present invention.

Referring now to FIG. 1, there is shown a photonelectron multiplier 9 incorporating features of the present invention and including an elongate cylindrical housing 10 of insulating material such as, for example, aluminum ceramic. The interior surface of the housing 10 is coated with a secondary-e]ectron-emitting material such as, for example, potassium chloride One end of the housing 10 is closed by a metallic anode disc 12 and the other end of the housing 10 is partially closed by a flat ring of insulating material such as, for example, aluminum ceramic, having a central aperture for receiving an optical fiber or waveguide 14 which projects axially within the housing 10. The optical fiber is of the type described in the article entitled Fiber Optics by Narinder S. Kapany in Scientific American, November 1960, pp. 72-81.

Outside the housing 10, the cylindrical surface of the fiber 14 is provided with'a coating of low refractive index material to produce internal reflections of light in the fiber, while inside the housing 10, the exterior cylindrical surface of the fiber 14 is coated with a semi-transparent light-absorbing photocathode, such as, for example, C5 0, Cs Sb or (NaKCs)Sb preferably having a refractive index higher than that of the optical fiber 14 in order to avoid problems of total reflection of light at the fiber-cathode interface 17. Of course, the refractive index of the cathode material can be the same or nearly the same as that of the fiber. The end of the optical fiber 14 within the housing 10 is silvered or coated with a dielectric coating 18 having a low refractive index for reversing the fall of light within the optical fiber 14.

The members making up the photomultiplier 9 are sealed together in a vacuum-type manner and the space between the photocathode 16 and the secondary emitter 11 evacuated. Potentials of respectively increasing mag nitude are applied to the photocathode 16, the secondaryemitter 11 and the anode 12. Typically, a potential difference of several hundred volts is established between the photocathode 16 and the adjacent portion of the secondary-emitter-electrode 11, and a potential difference of several thousand volts is established between the anode and the adjacent end of the secondary-emitting electrode, the secondary-emitting electrode having a positive potential gradient that varies from a few volts near the cathode to several hundred volts near the anode.

When light is introduced into the input end of the fiber 14 at angles of incidence up to the angle determined as set forth in greater detail below, the light is reflected from those walls of the fiber 14 outside the housing 10 so that the light is transmitted down the length of the fiber into the photomultiplier 9. Within the photomultiplier 9, part of the light striking the interface 17 between the fiber 14 and the cathode 16 enters and passes through the photocathode 16 to be absorbed for the production of electrons. When the light strikes the interface 19 between the cathode and the vacuum the light is totally reflected back through the cathode, as illustrated in greater detail below, and back through the fiber 14 for passage and absorption again in another portion of the photocathode 16. Since the fiber 14 is cylindrical, light is reflected back and forth within the fiber at the same incidence angle. However, the angle of incidence of the light on the inner end of the fiber 17 is the complement of the incident angle on the other surfaces of the fiber.

As light is transmitted through the photocathode, electrons are generated which are attracted to the secondaryemission-material 11 which, upon bombardment, generates secondary-electrons. Due to the graded potential on the secondary emitter, more and more electrons are generated in the photomultiplier in the direction of the anode 12 to produce an output amplified electrical signal in proportion to the amount of light incident on the input face of the fiber 14.

The parameters of the photomultiplier 9 are selected such that sufficient reflections occur to absorb substantially all of the light energy in the photocathode. Thus, the photomultiplier as described above has a high sensitivity and electronic gain over a broad frequency range, particularly advantageous in optical tracking systems. The photomultiplier 9 can be provided with a light input lens 8 which directs parallel light onto the fiber at the incident angle for which maximum absorption takes place or can be incorporated in a telescope structure when used in optical tracking systems as in a star tracker.

Referring now to FIG. 2, there is an alternative construction of a photomultiplier in accordance with the present invention. There, the photomultiplier includes a fiber 20, one end of which projects axially within the region between two parallel, spaced-apart-circular end plates 21 and 22. Coaxially mounted between the two end plates and surrounding the fiber are a pair of transmissive type cylindrical secondary-electron-emitter electrodes 24 and 25 and an anode 26, electrode 25 having a diameter greater than the electrode 24 and anode 26 having a diameter greater than electrode 25.

The cylindrical surface of the fiber 20 within the photomultiplier is covered with a photocathode 23 such as one of the types described above. Light introduced into the end of the fiber 20 at angles up to is multiply internally reflected within the fiber 20 and cathode 23 whereby substantially all of the energy of the light is absorbed in the cathode 23 to generate electrons which are in turn drawn to the transmissive secondary-electron-emitting electrode 24 which is maintained at a more positive potential than the photocathode 23. Electrons striking the electrode 24 create secondaryemission electrons at the outer surface of the electrode 24 which are in turn drawn to the electrode 25 for generation of more electrons to be collected at the anode 26. Typical transmissive secondary-electron-emitting materials are smoked MgO, BaF and KC] on thin aluminum foil. Naturally, other materials can be used.

The photomultiplier illustrated in FIG. 2 has a faster response than the multiplier shown in FIG. 1 due primarily to a faster transit time and the transit time dispersion is less. Additionally, the structure shown in FIG. 2 avoids the necessity for a graded potential on the electrodes and can be conveniently constructed using closely-spaced coaxial cylinders which provide a very short transit time for the initially emitted-electrons and the secondary-emitted-electrons.

Referring now to FIG. 3 for an explanation of the Operation of the invention, there is illustrated an enlarged view of a portion of the structure illustrated in FIG 1. Most of the light in a first medium with index n upon striking the input face of the fiber 14 with a higher index n, at angles of incidence up to the angle enters the fiber 14 and is refracted toward the normal. As will be seen below, the angle between the refracted light and the normal in the fiber 14 is -0 Also, as will be seen below, light incident on the input face 14, at angles of incidence greater than s would not be totally reflected at the cathode-vacuum interface 19.

The refracted light ray within the fiber 14 strikes the fiber-cathode interface 17 at an angle of incidence 0 which is the complement of 90-0 since the input face of the fiber 14 is normal to cylindrical fiber-cathode interface 17. Since the photocathode material has a refractive index n which is higher than that of the fiber 14, much of the light incident on the fiber-cathode interface 17 is refracted toward the normal as it enters the cathode 16. The cathode 16, being semi-transparent and light absorbing, absorbs a portion of the light as the light passes therethrough and strikes the cathode-vacuum interface 19. The vacuum has an index of refraction n which may or may not be the same as n The angle of incidence is such that the angle of incidence 6 of the light at the fiber-cathode interface 17 is the critical angle or angle of total reflection for trapping the light in the cathode and not permitting passage of light into the vacuum beyond the cathode mission surface. At the critical angle 0 much of this light is retracted, and instead of passing into the vacuum, travels along the interface 19 for energy exchange with the photocathode material to produce electrons.

Naturally each time light strikes any of the interfaces mentioned above a certain amount of light is reflected at a reflection angle equal to the incidence angle. As illustrated in the drawing, ultimately substantially all of the light which initially enters the fiber 14 at the input end finds its way into the cathode for absorption.

In order to determine an expression for qi for this phototube, we utilize Snells law which states that the product of the refractive index and the sine of the angle of incidence of light in one medium equals the product of the refractive index and the sine of the angle of refraction in the medium into which the light travels. Thus, it is seen that n; sin 9 =n sin 90, or 0 =sin'- n /n Again, it is seen that n sin =n sin (900 or rnnx= Sill no and where n =n 2 2)1/2 max= SIITIT As long as the angle of incidence at the input face of the fiber 14 is equal to or less than M total internal reflection takes place. Thus, if the angle of incidence at the input face of fiber 14 is less than qfi the angle of refraction in the fiber 14 is less than 900,, and the angle of incidence at the fiber-cathode interface is greater than 0,.

For an axial cone of radiation of half-angle incident on the fiber, the total flux at wavelength A absorbed by the photocathode will be:

where I /21r=incident flux per unit solid angle at wavelength x n-number of refiections=L tan d L=length of the coated fiber d==fiber diameter R()\,)=fracti0n of flux at wavelength A and angle which is reflected by the photocathode at each pass across the fiber substrate If the photon-electron quantum efficiency is I'()\) then the total number of photoelectrons produced will be:

In a typical photomultiplier in accordance with this invention, the inherent optimum photocathode thickness is approximately 250 A., and this photocathode coating is provided on a fiber such as, for example, 0.005" in diameter. In a multiple total reflection type photocathode of this construction, red light is substantially entirely absorbed after six reflections whereas blue light is substantially entirely absorbed after only two reflections.

Referring now to FIG. 4, there is shown a photomultiplier embodying features of the present invention and including a first medium such as, for example, glass, which is provided on one flat surface thereof with a layer of photocathode material 31 such as any one of the photocathode materials mentioned above. Mounted in front of the photocathode 31 within an evacuated housing 32 are a plurality of transmissive type secondaryelectron-emitting electrodes 33 and 34 maintained at progressively higher potentials than the photocathode 31. A low index material or metallic coating such as, for example, silver, is provided on the backing surface of the medium 30 parallel to and opposing the photocathode 31 for reflecting back onto the photocathode light initially reflected from the photocathode.

Light is introduced into the first medium 30 at the proper angle such that light transmitting through the first medium into the photocathode 31 at an angle equal to or greater than the critical angle at the cathode interface for reflecting and refracting light into the cathode materhifor absorption and for additional reflections at the photocathode-vacuum interface.

Light energy traveling through the photocathode 31 produces at the surface of the photocathode 31 electrons which are drawn to the secondary-electron-emitting electrode 33 which inturn, upon bombardment, produces additional secondary electrons which bombard the electrode 34 and so on to an anode not shown. The coating 35 insures reflection of light from the photocathode back to the photocathode for additional absorption at the photocathode whereby multiple light reflections take place to absorb substantially entirely all of the light energy in the photocathode. Thus, even for weak input light signals, a substantial electrical signal is produced in the photomultiplier.

Referring now to FIG. 5, there is shown an alternative structure embodying features of the present invention and in which the first medium on which the photocathode 41 is supported is constructed to receive light over a wide range of incident angles and still produces multiple internal reflections to absorb substantially all of the input light energy in the photocathode material. More specifically, the backing surface 46, instead of being parallel to the front surface on which the cathode is mounted, is provided with a reflector surface in the form of a mosaic lenticular or spherical reflector surface so that, regardless of the initial different angles of incidence, light striking the lenticular surface 46 after the first reflection from the photocathode 41 is redirected onto the photocathode 41 at an angle which is equal to or greater than the critical angle. Because of the configuration of surface 46, this light incident relationship at the cathode remains true after a number of reflections back and forth between the photocathode and the backing surface 46 to insure multiple reflections in the photocathode in order to absorb substantially all of the incident light energy in the photocathode for the production of the maximum number of electrons in the photomultiplier.

Still another alternative construction is illustrated in FIG. 6 in which the first medium 50 is provided with a total reflecting prism at the light input region having a lenticular or spherical surface 56 to collimate incident 8 non-collimated light, whereby the light directed onto the face of the first medium 50 on which the photocathode 51 is mounted is collimated light at an angle such that light transmitted into the photocathode reaches the cathodevacuum interface at an angle equal to or greater than the critical angle at that interface.

The constructions illustrated in both FIGS. 5 and 6 permit the utilization of an optical fiber to transmit light into the photomultiplier. Since the light output from the optical fiber is not collimated, the constructions illustrated in FIGS. 5 and 6 account for non-collimated light input to produce multiple reflections in the cathode so that substantially all of the input light is absorbed in the photocathode for the production of electrons.

Referring now to FIG. 7, there is illustrated utilization of the present invention in an optical tracking system such as a star tracker. The photomultiplier illustrated in FIG. 7 includes a plurality such as, for example, four optical fibers 61 housed within a plurality of transmissive type concentric secondary-electron'emitting electrodes 62 and 63 and a surrounding concentric cylindrical anode 64. The electrodes 62 and 63 and the anode 64 are divided into quadrants by radial optical and electrical insulating members 65 such as, for example, alumina ceramic vanes. The fibers 61 are also separated by the vanes 65 so that only one fiber 61 lies within each quadrant.

In each of the quadrants, the surface of the optical fiber 61 facing the adjacent secondary-electron-emitting electrode 62 is coated with a photocathode material 66 of one of the types described above whereas the remaining portion of the fiber is coated with a low refractive index material for producing internal reflections in the fiber.

The light input ends of the fiber 61 are closely packed together to form a light-receiving surface. Input light such as that received from a star through a telescope is focused on the input ends of the fibers and enters one of the fibers in the same manner as light entering the fiber illustrated in FIGS. 2 and 3. This light is transmitted down the fiber 61 into the photomultiplier in which multiple reflections take place in the cathode to absorb substantially all of the input light energy. Light reflected out of the photocathode material 66 back into the fiber 61 is reflected at the surface of the fiber at coating 67 to redirect the light back into the photocathode for absorption of additional energy.

The light energy absorbed in the photocathode material produces electrons which are drawn to the electrode 62 to produce secondary electrons and ultimately produce an output signal at the anode segment indicative of light input in the particular optical fiber in the particular quadrant in which the signal is produced. As the incident light is directed into a different fiber 61, the signal produced in one quadrant dies out and a signal is produced in the other quadrant into which light is directed. By connecting the output signals from the different quadrants to a guidance system, an optical tracker is produced which homes in on the light source at which it is directed.

While the optical transmission medium on which the photocathode is deposited in the construction illustrated in FIG. 6 is a cylinder, it is obvious that a medium of other cross-sectional configurations can be utilized. For example, the optical receiving element can be a single optical fiber or circular optical Waveguide divided into quadrants by a low index material or metallic coating so that the optical medium within the photomultiplier on which the photocathode is supported is a generally right angular prism having a semi-cylindrical diagonal surface on which the cathode material is supported. It will be seen, however, that with such a construction, the same angle of incidence onto the photocathode material for light reflected from the other walls of the prism will not always be repeated in order to always maintain the angle of incidence of light on the photocathode at or equal to the critical angle for trapping light in the cathode.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it is understood that certain changes and modifications may be practiced within the spirit of the invention as limited only by the scope of the appended claims.

In the claims:

1. A phototube comprising a first light transmitting medium having a given refractive index; a light transmitting and absorbing cathode layer bounding at least a portion of said first medium; and anode spaced from said cathode; a second medium between said cathode and said anode having a refractive index lower than said index of said first medium; means for establishing a potential difference between said cathode layer and said anode; means for introducing light into said first medium to penetrate said cathode and enter the cathode at an angle at least as great as the trapping angle which prevents light from passing into said second medium, the boundaries of said first medium arranged to redirect light that is reflected from said cathode back to said cathode at an angle at least as great as said trapping angle.

2. A phototube comprising an optical waveguide having a given refractive index; a light transmitting and absorbing cathode layer surrounding at least a portion of said waveguide; an anode spaced from said cathode; a medium between said cathode and said anode having a refractive index lower than said index of said optical waveguide; means for establishing a potential difference between said cathode layer and said anode; means for introducing light into said waveguide to penetrate said cathode and strike the interface between said waveguide and said cathode at an angle at least as great as the critical angle that prevents light from passing into said medium, the boundaries of said waveguide arranged to redirect light that is initially reflected from said cathode back to said cathode at an angle at least as great as said critical angle.

3. A phototube comprising a light transmitting and absorbing cathode having a substantially cylindrical surface and having a given refractive index; an anode spaced from said cathode; a medium between said cathode and said anode having a refractive index lower than said index of said cathode; means for applying potential to said cathode and said anode; means for directing light into said cathode onto the interface between said cathode and said medium at an angle at least as great as the critical angle at said interface; and means for redirecting light initially reflected from said interface back onto said interface at an angle at least as great as said critical angle.

4. A phototube comprising a first light transmitting medium having a given rafractive index; a light transmitting and absorbing cathode layer surrounding at least a portion of said first medium; an anode spaced from said cathode; a second medium between said cathode layer and said anode having a refractive index lower than said index of said first medium; means for establishing a potential difference between said cathode layer and said anode; means for introducing light into said first medium at an angle or less to penetrate said cathode and strike the interface between said cathode and said second medium without passing into said second medium, the angle determined from the formula where n is the refractive index of said first medium, 11 is the refractive index of said optical waveguide and n is the refractive index of said second medium, the boundaries of said first medium being arranged to redirect light that is initially reflected from said cathode back to said cathode.

5. A photomultiplier comprising a first light transmitting medium having a given refractive index; a light transmitting and absorbing cathode layer bounding at least a portion of said first medium; a secondary electron emitting electrode spaced from said cathode; a second medium between said cathode and said secondaryelectron-emitting electrode having a refractive index lower than said index of said first medium; means for establishing a potential difference between said cathode layer and said secondary electron emitting electrode; and means for introducing light into said first medium to penetrate said cathode and strike the interface between said first medium and said cathode at an angle at least as great as the critical angle that prevents light from passing into said second meduim, the boundaries of said first medium arranged to redirect light that is initially reflected from said cathode back to said cathode at an angle at least as great as said critical angle.

6. A photomultiplier according to claim 5 characterized further in that the interface between said cathode layer and said second medium is a cylindrical surface and said secondary-electron-emitting electrode has a cylindircal surface surrounding said cathode.

7. A photomultiplier in accordance with claim 5 characterized further in that said interface between said cathode layer and said second medium is a cylindrical surface and a cylindrical anode surrounds said cathode, said secondary emitting electrode being made of a transmissive secondary electron emitter positioned between said cathode and said anode.

8. A photomultiplier in accordance with claim 5 characterized further in that said interface between said cathode and said second medium is a cylindrical surface and said secondary-electron-emitting electrode is a cylindrical member surounding said cylindrical interface and including an anode positioned at one end of said secondaryelectron-emitting electrode for receiving electrons emitted from said secondary-emitting electrode and said cathode.

9. A photomultiplier comprising a light transmitting and absorbing cathode having a hollow light receiving portion; an anode; a secondary emitting electrode; said cathode being formed of a material to emit electrons when the cathode is subjected to light energy and positioned to bombard said secondary-emitting electrode with electron bombardment when subject to light energy; said secondary-emitting electrode being disposed to emit electrons to said anode when energized by electron bombardment from said cathode; means supplying potential to said anode, said secondary-emitting electrode, and said cathode; a medium separating said cathode from said secondary-emitting electrode and said anode; said cathode having a refractive index greater than the refractive index of said medium; and means for interjecting light into said light-receiving portion of said cathode to strike the interface of said cathode and said medium such that substantially no light passes into said medium, and means for directing light reflected from said interface back onto said inter-face such that substantially no light passes into said medium.

10. A phototube comprising an optical waveguide; a light transmitting and absorbing cathode layer surrounding at least a portion of said waveguide and having a hollow light receiving portion; an anode; said cathode being formed of a material emitting electrons when subject to light energy and an electric field; means for applying a potential difference between said anode and said cathode layer; a medium separating said cathode from said anode; an electron emitting face on said cathode formed in a tubular configuration and forming a tubular interface with said medium; means for interjecting light into said optical waveguide and said light 11 receiving portion of said cathode to strike said face without passing into said medium.

11. A phototube comprising a light transmitting and absorbing cathode having a light input side and an electron emitting output side, said electron emitting output side being formed in a substantially tubular configuration; an anode mounted to receive electrons emitted from said cathode; and electrical insulating medium separating said cathode and said anode; means to establish a voltage between said cathode and anode to cause electron fiow from said cathode to said anode 'when said cathode is subjected to .light energy; and

means for introducing light into said cathode and onto the interface between said cathode and said separating medium without passing into said medium; and means for redirecting light initially reffected from said interface back onto said interface without passing into said medium.

12. A phototube comprising an optical fiber having a light receiving end; a light transmitting and absorbing cathode layer surrounding at least a portion of said optical fiber, said cathode being formed of a material to emit electrons when subjected to light energy and an electric field; an anode; a medium between said cathode and said anode; said medium having a refractive index lower than the refractive index of said fiber; means supplying a potential difference between said anode and said cathode, and means for introducing light into said light receiving end of said optical fiber to strike the interface between said fiber and said cathode at an angle at least as great as the critical angle to penetrate said cathode without passing into said medium, the length of the cathode layer surrounding said optical fiber be- 12 ing such that light introduced into said optical fiber will strike the interface between said fiber and said cathode at an angle at Jeast as great as said critical angle a plurality of times.

13. A phototube comprising a first light transmitting medium having a given refractive index divided into a plurality of segments; a plurality of light transmitting and absorbing cathode layers each bounding at least a portion of one of said segments of said first medium; anode segments spaced from each of said cathode layers; a second medium between said cathode layers and said anode segments having a refractive index lower than said refractive index of said first medium; means for establishing a potential difference between said cathode layers and said anodes; means for introducing light into each of said segments of said first medium to penetrate the respective cathode layers and strike the interface between said cathode layers and said second medium without passing into said second medium, the boundaries of said first medium segments arranged to redirect light that is refiected from said interface back onto said interface again without passing into said second medium.

References Cited by the Examiner UNITED STATES PATENTS 3,043,976 7/1962 Kossel 313-94 3,047,867 7/1962 McNaney 250-227 3,088,037 4/1963 Baum 88-1 JAMES W. LAWRENCE, Primary Examiner.

R. JUDD, Assistant Examiner. 

1. A PHOTOTUBE COMPRISING A FIRST LIGHT TRANSMITTING MEDIUM HAVING A GIVEN REFRACTIVE INDEX; A LIGHT TRANSMITTING AND ABSORBING CATHODE LAYER BOUNDING AT LEAST A PORTION OF SAID FIRST MEDIUM; AND ANODE SPACED FROM SAID CATHODE; A SECOND MEDIUM BETWEEN SAID CATHODE AND SAID ANODE HAVING A REFRACTIVE INDEX LOWER THAN SAID INDEX OF SAID FIRST MEDIUM; MEANS FOR ESTABLISHING A POTENTIAL DIFFERENCE BETWEEN SAID CATHODE LAYER AND SAID ANODE; MEANS FOR INTRODUCING LIGHT INTO SAID FIRST MEDIUM TO PENETRATE SAID CATHODE AND ENTER THE CATHODE AT AN ANGLE AT LEAST AS GREAT AS THE TRAPPING ANGLE WHICH PREVENTS LIGHT FROM PASSING INTO SAID SECOND MEDIUM, THE BOUNDARIES OF SAID FIRST MEDIUM ARRANGED TO REDIRECT LIGHT THAT IS REFLECTED FROM SAID CATHODE BACK TO SAID CATHODE AT AN ANGLE AT LEAST AS GREAT AS SAID TRAPPING ANGLE. 